C6 is a sentient C-62 steam locomotive, historically one of Japan's largest and fastest steam passenger locomotives. Possessing self-awareness and the unusual, unexplained ability to verbally communicate with animals, C6 befriends Yamataro, a young bear chained outside a store near the tracks. Acting as a mentor, C6 encourages Yamataro's escape, advising the bear to break free at night and wait for the town's harbor parade festival when humans would be absent. During this time, C6 coaches Yamataro to imitate his distinctive steam whistle, a skill bringing the bear minor local fame, offering both practical guidance and emotional support.

On the festival day, C6 transports Yamataro along the tracks towards the forest. However, the tracks end before reaching the bear's mountain sanctuary, forcing a separation. C6 bids a tearful farewell as Yamataro continues alone. Much later, C6 is discovered rusted and dust-covered. Humans restore him to operational condition specifically to use his recognizable whistle as bait to lure Yamataro back into captivity.

Faced with a critical choice, C6 must decide between cooperating with the hunters to secure his own preservation—avoiding the scrapyard for potential museum placement—or sacrificing himself to save Yamataro. C6 chooses self-sacrifice. He deliberately derails himself to block the path of the hunters' train, preventing them from reaching Yamataro. This heroic act causes severe damage leading directly to C6's demise; he is either destroyed in the wreck or hauled to a breaking yard and scrapped. His actions ensure Yamataro's successful escape to freedom in the mountains.

Personality-wise, C6 demonstrates profound loyalty and protective instincts towards Yamataro. He exhibits courage and resourcefulness in aiding the bear's escape and ultimately prioritizes his friend's safety over his own survival and potential future. His willingness to sacrifice himself for Yamataro's freedom defines his character arc.